The Circulating Hormone Concentration Is Regulated

The occupancy of hormone receptors can fluctuate greatly and is ultimately determined by the concentration of free hormone in the blood. The major determinants of hormone concentrations are (1) the rate of hormone secretion from endocrine cells and (2) the rate of hormone removal by clearance or metabolic inactivation. As we have seen, most hormones (with the exception of steroids) are stored in secretory granules. When the hormone is needed, the granule membranes fuse with the plasma membrane to liberate their contents into the bloodstream. This event is triggered by signals from other hormones or by neural signals. Stimulation of hormonal secretion is usually coupled with an increase of hormone synthesis, so that hormonal stores are replenished. 

Most hormones have a half-life in the blood of only a few minutes because they are cleared or metabolized very rapidly. The rapid degradation of hormones allows target cells to respond transiently. Polypeptide hormones are removed from the circulation by serum and cell surface proteases, by endocytosis followed by lysosomal degradation, and by glomerular filtration in the kidney. Steroid hormones are taken up by the liver and metabolized to inactive forms, which are excreted into the bile duct or back into the blood for removal by the kidneys. Catecholamines are metaboli-cally inactivated by O-methylation, by deamination, and by conjugation with sulfate or glucuronic acid. 

Thyroid hormones and most steroid hormones are associated with carrier proteins in the serum. The carrier proteins are called, appropriately, thyroxine-binding globulin, transcortin (for cortisol), and sex-steroid-binding protein. These proteins have a high affinity (Kd 10—9— 10 8 m) for their respective hormones. They buffer the concentration of free hormone and retard hormone degradation and excretion. The carrier proteins are distinguishable from the intracellular receptors for these hormones. 

---

Steroid Hormones Are Derived from Cholesterol The Circulating Hormone Concentration Is Regulated Hormone Action Is Mediated by Receptors... 

---